feat(P01): orca init full bootstrap + schema 0006

orca init transforms from a bare mkdir into a full single-node cluster
bootstrap. After `orca init`, `orca doctor` passes with zero FAILs
on the bootstrap checks (CA, cert, db, localhost node).

Changes:
- migration 0006: nodes.kind + nodes.os nullable columns (REQ-049)
- model.Node: Kind + OS fields + NodeKind constants (localhost|linux|proxmox)
- NodeRepo: extended Insert/Get/List/Watch/scanNode for kind/os columns
  (NULL -> "" mapping); added GetByName + UpdateLastSeenAndOS helpers
- internal/cli/osdetect.go: detectOS() from /etc/os-release ID= field
  (D-032); fallback to /usr/lib/os-release then "linux"
- internal/cli/init.go: full bootstrap sequence (REQ-047, REQ-048):
  1. MkdirAll namespace dir
  2. store.Open (runs migrations 0001..0006)
  3. security.CAInit (idempotent fast-path)
  4. server cert gen if absent (D-036: skip if present)
  5. detectOS from /etc/os-release
  6. localhost node upsert (insert if new, refresh last_seen+os if exists)
  Idempotent re-run: no duplicate node, no cert regen, id/joined_at preserved
- --json output: full bootstrap summary (namespace, db, ca_fp, cert_fp,
  os, node_id, steps array)
- tests: init idempotency, osdetect parsing (ubuntu/debian/alpine/pve),
  kind/os round-trip, NULL->"" mapping, GetByName, UpdateLastSeenAndOS

E2E smoke test: orca init -> 5 PASS / 0 WARN / 1 FAIL (network=daemon
not running, expected); orca node list shows localhost node (os=ubuntu).

---ci---
project: orca
phase: 1
milestone: v0.6
status: execute
---/ci---

.ciagent/CHECKPOINT.json

@@ -1,11 +1,11 @@
 {
-  "phase": 0,
-  "stage": "plan",
+  "phase": 1,
+  "stage": "execute",
   "milestone": "v0.6",
   "milestone_slug": "node-bootstrap-proxmox",
-  "phase_role": "pre_execution",
+  "phase_role": "execution",
   "attempts": 0,
-  "updated_at": "2026-08-03T20:45:00Z",
+  "updated_at": "2026-08-03T19:48:00Z",
   "milestone_complete": false,
   "next_milestone": null
 }